The aims of this study are: 1) To determine the prevalence of decreased BMD in children undergoing allogeneic BMT 2) To correlate the BMD with duration and dose of steroid and cyclosporine use (which is correlated with duration of GVHD), adjusting for other determinants of BMD, such as obesity, dietary intake of calcium and vitamin D, pubertal stage and physical activity 3) To measure the changes in bone mineralization over time and post-steroid/cyclosporine use 4) To assess the incidence of avascular necrosis (AVN) and fractures in these patients compared to age and gender-specific AVN and fracture incidence reference data. If this study confirms that BMD is diminished in a sample population of children post BMT, and either the BMD does not recover, or that the incidence of bone complications is increased, a randomized clinical trial of therapy for optimizing bone mass in children at risk is warranted.